| /* |
| * Copyright (C) 2014 The Android Open Source Project |
| * Copyright (c) 1997, 2017, Oracle and/or its affiliates. All rights reserved. |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. Oracle designates this |
| * particular file as subject to the "Classpath" exception as provided |
| * by Oracle in the LICENSE file that accompanied this code. |
| * |
| * This code is distributed in the hope that it will be useful, but WITHOUT |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
| * 2 along with this work; if not, write to the Free Software Foundation, |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| * |
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| */ |
| |
| |
| package java.lang.ref; |
| |
| |
| /** |
| * Abstract base class for reference objects. This class defines the |
| * operations common to all reference objects. Because reference objects are |
| * implemented in close cooperation with the garbage collector, this class may |
| * not be subclassed directly. |
| * |
| * @author Mark Reinhold |
| * @since 1.2 |
| */ |
| |
| @SuppressWarnings({"unchecked", "deprecation", "all"}) |
| public abstract class Reference<T> { |
| |
| Reference() { throw new RuntimeException("Stub!"); } |
| |
| /** |
| * Returns this reference object's referent. If this reference object has |
| * been cleared, either by the program or by the garbage collector, then |
| * this method returns <code>null</code>. |
| * |
| * @return The object to which this reference refers, or |
| * <code>null</code> if this reference object has been cleared |
| */ |
| |
| public T get() { throw new RuntimeException("Stub!"); } |
| |
| /** |
| * Clears this reference object. Invoking this method will not cause this |
| * object to be enqueued. |
| * |
| * <p> This method is invoked only by Java code; when the garbage collector |
| * clears references it does so directly, without invoking this method. |
| */ |
| |
| public void clear() { throw new RuntimeException("Stub!"); } |
| |
| /** |
| * Tells whether or not this reference object has been enqueued, either by |
| * the program or by the garbage collector. If this reference object was |
| * not registered with a queue when it was created, then this method will |
| * always return <code>false</code>. |
| * |
| * @return <code>true</code> if and only if this reference object has |
| * been enqueued |
| */ |
| |
| public boolean isEnqueued() { throw new RuntimeException("Stub!"); } |
| |
| /** |
| * Adds this reference object to the queue with which it is registered, |
| * if any. |
| * |
| * <p> This method is invoked only by Java code; when the garbage collector |
| * enqueues references it does so directly, without invoking this method. |
| * |
| * @return <code>true</code> if this reference object was successfully |
| * enqueued; <code>false</code> if it was already enqueued or if |
| * it was not registered with a queue when it was created |
| */ |
| |
| public boolean enqueue() { throw new RuntimeException("Stub!"); } |
| |
| /** |
| * Ensures that the object referenced by the given reference remains |
| * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>, |
| * regardless of any prior actions of the program that might otherwise cause |
| * the object to become unreachable; thus, the referenced object is not |
| * reclaimable by garbage collection at least until after the invocation of |
| * this method. Invocation of this method does not itself initiate garbage |
| * collection or finalization. |
| * |
| * <p> This method establishes an ordering for |
| * <a href="package-summary.html#reachability"><em>strong reachability</em></a> |
| * with respect to garbage collection. It controls relations that are |
| * otherwise only implicit in a program -- the reachability conditions |
| * triggering garbage collection. This method is designed for use in |
| * uncommon situations of premature finalization where using |
| * {@code synchronized} blocks or methods, or using other synchronization |
| * facilities are not possible or do not provide the desired control. This |
| * method is applicable only when reclamation may have visible effects, |
| * which is possible for objects with finalizers (See |
| * <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.6"> |
| * Section 12.6 17 of <cite>The Java™ Language Specification</cite></a>) |
| * that are implemented in ways that rely on ordering control for correctness. |
| * |
| * @apiNote |
| * Finalization may occur whenever the virtual machine detects that no |
| * reference to an object will ever be stored in the heap: The garbage |
| * collector may reclaim an object even if the fields of that object are |
| * still in use, so long as the object has otherwise become unreachable. |
| * This may have surprising and undesirable effects in cases such as the |
| * following example in which the bookkeeping associated with a class is |
| * managed through array indices. Here, method {@code action} uses a |
| * {@code reachabilityFence} to ensure that the {@code Resource} object is |
| * not reclaimed before bookkeeping on an associated |
| * {@code ExternalResource} has been performed; in particular here, to |
| * ensure that the array slot holding the {@code ExternalResource} is not |
| * nulled out in method {@link java.lang.Object#finalize Object#finalize}, which may otherwise run |
| * concurrently. |
| * |
| * <pre> {@code |
| * class Resource { |
| * private static ExternalResource[] externalResourceArray = ... |
| * |
| * int myIndex; |
| * Resource(...) { |
| * myIndex = ... |
| * externalResourceArray[myIndex] = ...; |
| * ... |
| * } |
| * protected void finalize() { |
| * externalResourceArray[myIndex] = null; |
| * ... |
| * } |
| * public void action() { |
| * try { |
| * // ... |
| * int i = myIndex; |
| * Resource.update(externalResourceArray[i]); |
| * } finally { |
| * Reference.reachabilityFence(this); |
| * } |
| * } |
| * private static void update(ExternalResource ext) { |
| * ext.status = ...; |
| * } |
| * }}</pre> |
| * |
| * Here, the invocation of {@code reachabilityFence} is nonintuitively |
| * placed <em>after</em> the call to {@code update}, to ensure that the |
| * array slot is not nulled out by {@link java.lang.Object#finalize Object#finalize} before the |
| * update, even if the call to {@code action} was the last use of this |
| * object. This might be the case if, for example a usage in a user program |
| * had the form {@code new Resource().action();} which retains no other |
| * reference to this {@code Resource}. While probably overkill here, |
| * {@code reachabilityFence} is placed in a {@code finally} block to ensure |
| * that it is invoked across all paths in the method. In a method with more |
| * complex control paths, you might need further precautions to ensure that |
| * {@code reachabilityFence} is encountered along all of them. |
| * |
| * <p> It is sometimes possible to better encapsulate use of |
| * {@code reachabilityFence}. Continuing the above example, if it were |
| * acceptable for the call to method {@code update} to proceed even if the |
| * finalizer had already executed (nulling out slot), then you could |
| * localize use of {@code reachabilityFence}: |
| * |
| * <pre> {@code |
| * public void action2() { |
| * // ... |
| * Resource.update(getExternalResource()); |
| * } |
| * private ExternalResource getExternalResource() { |
| * ExternalResource ext = externalResourceArray[myIndex]; |
| * Reference.reachabilityFence(this); |
| * return ext; |
| * }}</pre> |
| * |
| * <p> Method {@code reachabilityFence} is not required in constructions |
| * that themselves ensure reachability. For example, because objects that |
| * are locked cannot, in general, be reclaimed, it would suffice if all |
| * accesses of the object, in all methods of class {@code Resource} |
| * (including {@code finalize}) were enclosed in {@code synchronized (this)} |
| * blocks. (Further, such blocks must not include infinite loops, or |
| * themselves be unreachable, which fall into the corner case exceptions to |
| * the "in general" disclaimer.) However, method {@code reachabilityFence} |
| * remains a better option in cases where this approach is not as efficient, |
| * desirable, or possible; for example because it would encounter deadlock. |
| * |
| * @param ref the reference. If {@code null}, this method has no effect. |
| * @since 9 |
| */ |
| |
| public static void reachabilityFence(java.lang.Object ref) { throw new RuntimeException("Stub!"); } |
| } |
| |
